This invention is directed to improving the performance of high temperature resistant metal honeycomb core structural panels by localized densification of the panel.
Strong, lightweight structural boards have long been made for use in doors, walls, and the like. Generally, such boards consist of two impervious sheets or skins with a lightweight material bonded between the boards. Typical of these are the doors described by Konstandt in U.S. Pat. No. 2,633,439 where spiraled wood shavings are packed between plaster or resin sheets and the assembly is bonded together. A similar panel is described in German patent 1175861, where small coiled members are coated with adhesive and packed between face sheets to form a light weight door. While suitable for some applications, these panels have relatively low strength, not incorporating the more efficient honeycomb core structure and are not resistant to high temperatures.
Lightweight, high strength, panels made up of a metal honeycomb core with one or two metal face sheets bonded thereto have come into widespread use in aircraft, space vehicles and the like. For many purposes, high temperature resistant panels with the components bonded by brazing, diffusion bonding or the like, are necessary. Many presently known panels do not have the required high temperature resistance. Also, while some panels have great strength across a broad panel, difficulties are encountered when it becomes necessary to fasten other structures to the panels or where locally higher forces are directed against the panel. Often it is necessary to bond external structures to areas of the honeycomb panel. Such attachments tend to apply localized pressure, loads and stress to the panels. Without reinforcement the panels are subject to crushing when highly localized stress is applied.
In the manufacture of panels having honeycomb or circular cell cores, a coiled strip is sometimes placed in each desired cell as a manufacturing aid. As described by Lorentzen in Canadian Patent 680,648, a foil coil maybe placed in each honeycomb cell during bonding of cell interfaces together, so as to prevent bonding in regions intended to form the open cell. These coils are placed in every cell, so that localized reinforcement is not possible. While the cells do increase strength slightly across the panel, in areas that do not require reinforcement they merely add unnecessary and usually undesirable weight. Further, this manufacturing technique is useful only with adhesively bonded honeycomb structures which do not have the often required high temperature resistance.
Somewhat similarly, Evans et al in British Patent 2,197,618 describe placing scrolled paper tubes in circular cells forming the core of a panel. The tubes are placed in all cells, not in localized areas. The tubes are intended to support the tubes during bonding of resin impregnated fiber cell walls. With carbon-carbon composites, the paper tubes are charred and later removed, so that no permanent reinforcement is provided.
Also, while the thermal insulating properties of honeycomb panels are highly desirable in many applications, in some cases it is beneficial to improve heat transfer in local area from a heat source on one side of the panel to heat sink or the like on the other side of the panel. Most honeycomb panels have only thin webs spanning the distance between face sheets, providing very limited heat transfer.
In the past, attempts have been made to improve the local strength and thermal transfer characteristics of honeycomb panels by cutting holes in the panel face sheet and injecting a synthetic resin potting compound or inserting a resin insert to fill selected cells or groups of cells, as described in German patent 2425014. A fastener or the like may be embedded in the potting compound, as described by Moji et al in U.S. Pat. No. 4,716,067. While effective for some purposes, this method is not adaptable to all fasteners, requires considerable hand-work and care to properly fill the cells and does not improve thermal conductivity, since most potting materials have low thermal conductivity these materials seriously deteriorate at the high temperatures required in many aerospace applications.
As shown in British Patent 794,217, honeycomb cells may be reinforced by inserting adhesive coated tubes into selected cells. The tubes seem to be a loose fit with an exterior coating of adhesive filling the gap between tube and interior cell wall. Uniform pressure contact between tube and cell wall does not seem possible, since inserting a tight-fitting tube could easily damage the thin cell walls. Even if cells and tube are formed from a suitable metal, the use of adhesive bonding would preclude use at high temperatures. Also, if the cell shape is distorted in handling, as often happens, the tubes will not fit well.
In other cases, a region of the honeycomb core is removed, prior to application of the face sheets, and a solid metal insert is placed in the cutout. The insert is typically bonded to the face sheets when those sheets are bonded to the core. While these solid inserts are effective with a great many fasteners and have excellent thermal conductivity, they tend to be heavy and add an undesirably amount of weight, which can be of critical importance in aircraft and space vehicle applications. In many cases only small improvements in strength of the local panel area is necessary so that the greater improvement provided by solid inserts is not necessary.
Therefor there is a continuing need for improved honeycomb core panels with selectively located inserts which can increase the density, strength and thermal conductivity to a variable selected extent in local areas without excessively increasing the weight or complexity of the assembly or compromising the ability to use the panel at high temperatures.